Recently, with mass production, automation, and refinement of production facilities, functions which rely on the naked eye of a person or various types of sensors have been increasingly replaced by vision machines including image sensors. Charge-coupled devices (CCDs) are semiconductor devices that are mainly used in these image sensors.
A CCD refers to a device capable of transmitting a charge from one device to another adjacent device. An image sensor including such CCDs has a structure in which a change of an amount of free charges in each cell due to an amount of light is converted into an electrical signal.
Structurally, the CCD mainly includes a cell area in which charges are substantially accumulated due to an amount of light, and an output unit including a shift register which serves as a path through which the accumulated charges are sequentially transmitted.
The CCDs are divided into an area scan method, a line scan method, a time delay integration (TDI) line scan method, and the like according to a method in which cells are arranged in an array and an image is generated.
An image sensor using a line scan method (hereinafter referred to as a “line sensor”) is a one-dimensional sensor in which pixels which receive image light are arranged in a line. When a two-dimensionally widened image is imaged, a subject is sequentially imaged line by line by moving the line sensor or the subject.
That is, the line scan method is a method in which one line is exposed and transmitted at an arbitrary speed at a time, and has an advantage in that a high-speed and high-resolution image may be obtained with low cost compared to an area scan method. For example, while 4M pixels are required in the area scan method in order to obtain a frame having a size of 2048*2048, frames having various sizes such as 2048*2048, 2048*1000, and the like may be obtained using only 2K pixels in the line scan method.
However, when a high-speed scan method such as a case in which a subject which is moving at a high speed is imaged or a case in which a subject is imaged by moving a line sensor at a high speed is performed, since accumulation and transmission of charges at the high speed are repeated in each line, a time during which the charges may be accumulated per line is reduced, and thus an amount of light of the image becomes insufficient. Thus, demand for an amount of light is increasing, but the amount of light may not be indefinitely increased due to a limitation of a lighting device.
Accordingly, while research on increasing sensitivity by improving materials of a pinned-photodiode (PPD), a complementary metal-oxide-semiconductor (CMOS) sensor, and the like has been progressing, a method in which an amount of light is accumulated by arranging several line sensors to increase sensitivity has been proposed.
In an image sensor using a TDI line scan method (hereinafter referred to as a “TDI line image sensor”), line sensors are arranged as a plurality of stages in a scan direction, charges accumulated in CCDs in each line are synchronized with movement of an image, and are transmitted to CCDs in a next line. The charges overlap and are then output by repeating the process until the last line sensor, and as a result, an image having a sufficiently satisfactory amount of light may be obtained even in a high-speed scan method.
The related art of the present invention is disclosed in Korean Unexamined Patent Application Publication No. 2009-0023573 (published on Mar. 5, 2009, Invention Title: Method for Controlling a TDI-CCD Image Sensor).